1. Field of the Invention
The present invention relates to a CDMA reception apparatus and power control method therefor which are used for a mobile communication system and, more particularly, to a CDMA reception apparatus for performing RAKE reception and a power control method for the apparatus.
2. Description of the Prior Art
Conventionally, a mobile communication system using CDMA (Code Division Multiple Access) has been known.
In this CDMA mobile communication system, when data is to be transmitted from a mobile station to a base station, transmission data is transmitted after it is spread by using a corresponding one of spreading codes assigned to the respective mobile stations, and the base station demodulates the transmission data from each mobile station by despreading the data by using the spreading code assigned to each mobile station.
Likewise, data transmitted from a base station to a mobile station is also spread by a corresponding one of the spreading codes assigned to the respective mobile stations before it is transmitted. The resultant data is then transmitted.
In urban areas, in particular, various obstacles are present between a base station and a mobile station, and hence radio waves from the base station are often reflected by these obstacles and reach the mobile station. In such a situation, there are many reflected waves, which are reflected by various obstacles and reach the mobile station, between the base station and the mobile station as well as direct waves that directly reach the mobile station. That is, a so-called multi-path, in which there are a plurality of routes through which radio waves reach the mobile station, occurs.
The respective multi-path radio waves reach the mobile station with delay times corresponding to the respective routes. The mobile station therefore can improve reception quality owing to a path diversity effect by combining the multi-path radio waves in consideration of the delay times. This reception method will be referred to as a RAKE reception method.
A reception apparatus using this RAKE reception method needs to have fingers for despreading and the like equal in number to paths to a RAKE combiner. If, therefore, the number of paths to the RAKE combiner is too large, many fingers are required, resulting in increases in the size and cost of the apparatus. Considering that a mobile station moves all the time, the manner in which a multi-path occurs always changes. In some case, therefore, the use of too many fingers cannot allow the RAKE combiner to obtain a satisfactory reception quality improving effect, i.e., a path diversity effect.
For this reason, it is necessary to set the number of fingers to be set in the apparatus so as to obtain a reception quality improving effect by the RAKE combiner, i.e., a path diversity effect, to such an extent that the apparatus size does not increase too much.
In such a situation, in a CDMA reception apparatus having a limited number of fingers, the delay times in the respective fingers must be controlled to reliably capture multi-path radio waves.
As a conventional reception apparatus that solves this problem, the reception apparatus disclosed in Japanese Unexamined Patent Publication No. 9-181704 is available.
FIG. 1 is a block diagram showing the arrangement of a reception apparatus disclosed in Japanese Unexamined Patent Publication No. 9-181704. The first conventional apparatus will be described below with reference to FIG. 1.
Reference numeral 100 denotes a terminal to which a reception input spread signal is input; 200, a tracking finger for performing tracking and despreading; and 300, a search finger for detecting the level of a reception signal in each phase.
Reference numeral 402 denotes a RAKE combining path selecting section for selecting a phase of a spreading code in accordance with signals from the search finger 300 and tracking finger 200.
Reference numeral 403 denotes a pilot interpolation absolute synchronous detector for performing synchronous detection of the signal despread by the tracking finger 200.
Reference numeral 404 denotes a long code spreading code replica generator for supplying, to the tracking finger 200 or search finger 300, a spreading code replica corresponding to a specific channel to be used. The tracking finger 200 or search finger 300 uses this spreading code replica after delaying it by a predetermined amount through a spreading code replica delay section 206 or 305.
Reference numeral 405 denotes a RAKE combiner for combining signals from the respective paths; and 410, an output terminal.
Reference numerals 201, 202, 203, and 310 denote multipliers each serving to despread a reception signal by multiplying it by a spreading code replica; 204, 250, 207, and 302, integration dump circuits each serving to perform integration for a predetermined period of time; 208, 209, and 303, amplitude squaring circuits each serving to detect a signal level by performing amplitude squaring detection; and 304, a reception level memory for storing an output from the squaring circuit 303.
Reference numeral 210 denotes an adder for adding an output from the amplitude squaring circuit 208 to an output from the amplitude squaring circuit 209 with opposite polarities to generate a chip timing error signal associated with the spreading code replicas.
Reference numeral 211 denotes a loop filter for averaging the chip timing error signals from the adder 210 and outputting the resultant data. The data output from the loop filter 211 is input to a spreading code replica timing control signal generating section 212. In accordance with an output from the spreading code replica timing control signal generating section 212, the phase of the spreading code replica used by the RAKE combining path selecting section 402 for despreading.
The operation of the conventional technique shown in FIG. 1 will be described below.
The tracking finger 200 performs despreading by using a spreading replica code corresponding to a delayed path designated by the RAKE combining path selecting section 402 on the basis of the reception level detection information of all the chip phases of the search finger 300. The signal obtained by this despreading is demodulated.
As a demodulation scheme, delay detection, synchronous detection, or the like is available. In absolute synchronous detection, an absolute phase of reception must be estimated. In this prior art, the pilot interpolation absolute synchronous detector 403 performs absolute synchronous detection by estimating the phase of each information symbol by using a pilot signal and the phase of a pilot symbol as a reference phase.
In the tracking finger 200, the multipliers 201 and 202 perform correlation detection by using a reception spread/modulated signal and a replica code obtained by shifting a spreading code replica phase synchronized with the spreading code phase of a reception signal from each path by xc2x1xcex94 phase, and the integration dump circuits 204 and 205 performs integration for a predetermined period of time. The amplitude squaring circuits 208 and 209 then perform amplitude squaring detection to remove data modulation components and instantaneous phase variation components.
The adder 210 adds the amplitude square outputs of the spreading code replica obtained by the +xcex94 phase shift and the spreading code replica obtained by the xe2x88x92xcex94 phase shift with opposite polarities to generate a chip timing error signal associated with the spreading code replicas.
The loop filter 211 averages these chip timing error signals. The phase of the spreading code replica is updated in accordance with an output signal from the loop filter 211.
This phase update information is input to the RAKE combining path selecting section 402. The RAKE combining path selecting section 402 manages RAKE combining paths in real time to prevent overlaps between paths.
The RAKE combining path selecting section 402 updates the RAKE combining paths in predetermined cycles on the basis of an average delay profile of phase information search finger outputs of spreading code replicas corresponding to the respective paths to the RAKE combiner.
The RAKE combining path selecting section 402 generates spreading code replica signals for demodulation and generation of chip timing error signals in the tracking finger 200. The tracking finger 200 performs correlation detection of the spreading code replica corresponding to each path having this time delay and the input spread/modulated signal for a predetermined period of time, and inputs the integration output signal to the modulator, i.e., the pilot interpolation absolute synchronous detector 403.
According to the above reception apparatus disclosed in Japanese Unexamined Patent Publication No. 9-181704, to reliably capture multi-path radio waves with a limited number of fingers, the delay times of the respective fingers can be controlled.
The following problem is, however, posed in such a conventional reception.
The conventional reception apparatus is designed to use all fingers regardless of how a multi-path occurs. Assume that the number of paths in a multi-path is small, and hence a much effect cannot be obtained even by RAKE combining operation using many fingers. In this case, unnecessary fingers are operated to waste power.
As a conventional technique for solving such a problem, the receiver disclosed in Japanese Unexamined Patent Publication No. 7-231278 is available.
FIG. 2 is a block diagram showing the arrangement of the receiver disclosed in Japanese Unexamined Patent Publication No. 7-231278. The second conventional apparatus will be described below with reference to FIG. 2.
Reference numerals 501 to 50N denote the first to Nth spreading means for receiving direct spread/modulated signals with N paths and despreading/demodulating the respective direct spread/modulated signals S30 with the first to Nth spreading sequences synchronized with the N direct spread/modulated signals S30 received at different timings.
Reference numeral 51 denotes a combining means for combining output data D401 to D40N from the first to Nth despreading means 501 to 50N, and 52 represents a control means for obtaining the level difference between one of the N direct spread/modulated signals S30 which has the highest level and each of the remaining signals. If each of the obtained level differences is equal to or higher than a threshold T, the combining means 51 turns off despreading means (e.g., 50Nxe2x88x922, 50Nxe2x88x921, and 50N) for despreading/demodulating signals having level differences of equal to or higher than the threshold T with respect to the signal having the highest level.
A path diversity effect can be satisfactorily obtained by using a RAKE receiver when signals having levels similar to each other to some degree are input through a multi-path. In addition, if a signal having a level noticeably (greatly) lower than the levels of a group of signals having levels similar to each other to some degree is input, the obtained diversity effect is small.
For a RAKE receiver, therefore, importance is attached to the way to extract only a signal group that provides a great diversity effect.
In the method of obtaining level differences from the highest levels, as in the receiver disclosed in Japanese Unexamined Patent Publication No. 7-231278, however, it is very difficult to extract only a signal group that can provide a great diversity effect. The reason for this difficulty will be described below.
FIG. 3 is a graph showing multi-path signals arranged in the descending order of levels.
Referring to FIG. 3, the signal having the highest level is represented by L1, and signals having lower levels are sequentially represented by L2, L3, . . . . In addition, the level difference between the signals L1 and L2 is represented by a; the level difference between the signals L2 and L3, b; and the level difference between the signals L3 and L4, c.
In this case, the expression xe2x80x9ca signal group having levels similar to each other to some degreexe2x80x9d indicates the signals L1, L2, L3, and L4 in FIG. 3. Assume that in the receiver disclosed in Japanese Unexamined Patent Publication No. 7-231278, the relationship between a threshold T and each signal level is represented by a+b less than T and a+b+c greater than T. In this case, the signal L4 is discarded although this signal can provide a great path diversity effect. If such a situation occurs in the case of a weak electric field, a deterioration in reception quality cannot be avoided.
At paragraph number 0028 in Japanese Unexamined Patent Publication No. 7-231278, it is described that xe2x80x9cIn addition, the threshold T is preferably set to a value corresponding to the level difference between one of the direct spread/modulated signals S30, with which no path diversity effect can be obtained by synthesis, and the signal having the highest levelxe2x80x9d. In practice, however, the signal having the highest level always varies in level, and it is difficult to find xe2x80x9cone of the direct spread/modulated signals S30, with which no path diversity effect can be obtained by synthesisxe2x80x9d. For this reason, it is very difficult or impossible to determine the value of the threshold T as described at paragraph number 0028.
The present invention has been made in consideration of the above problems in the prior art, and has as its object to provide a CDMA reception apparatus designed to change the number of fingers to be used for RAKE reception in accordance with an occurrence state of multi-path, which can guarantee reception quality even in a weak electric field as much as possible, and reduce a power consumption by stopping operation of unnecessary fingers.
In order to achieve the above object, according to the first main aspect of the present invention, there is provided a CDMA reception apparatus having a plurality of fingers and serving to perform RAKE reception by RAKE-combining outputs from the plurality of fingers, comprising multi-path detecting means for detecting a multi-path consisting of a plurality of radio waves arriving with some delay times by despreading a reception signal, and delay profile generating means for generating a delay profile by obtaining reception power levels of all reception signals in the multi-path detected by the multi-path detecting means, wherein (a) a difference di in level between a signal having an ith highest level and a signal having an (i+1)th highest level of data of the delay profile is obtained, and operation of a finger corresponding to a signal having a low level is stopped on the basis of the difference di and a predetermined threshold dTH, (b) operation of a finger corresponding to a signal having a low level is stopped on the basis of a result obtained by comparing a threshold LTH with a level Li of a signal having an ith highest level of the data of the delay profile, or (c) if a level of a signal having a highest level of the data of the delay profile is not more than a predetermined threshold LMAX, operation of all fingers is stopped.
The present invention has the following minor aspects associated with the first main aspect.
The difference di in level between the signal having the ith highest level and the signal having the (i+1)th highest level of data of the delay profile is obtained, and operation of a finger corresponding to a signal having a level lower than that of the signal having the (i+1)th highest level is stopped if the difference di is not less than the predetermined threshold dTH.
In addition, if the difference di in level between the signal having the ith highest level and the signal having the (i+1)th highest level of data of the delay profile is not more than a predetermined value, and a difference di+1 in level between the signal having the (i+1)th highest level and a signal having an (i+2)th highest level is not less than the predetermined value, operation of a finger corresponding to a signal having a level lower than that of the signal having the (i+2)th highest level is stopped.
If the level Li of the signal having the ith highest level of the data of the delay profile is not more than the predetermined threshold LTH, operation of a finger corresponding to a signal having a level lower than that of the signal having the ith highest level is stopped.
The CDMA reception apparatus according to the main aspect further comprises a user input device by which a user can set the threshold.
The CDMA reception apparatus according to the main aspect further comprises display means for displaying information indicating the number of fingers, of the plurality of fingers, to which power is supplied.
Processing (c) in the main aspect is performed from the viewpoint that an improvement in reception characteristics takes priority over power saving in a weak electric field.
In order to achieve the above object, according to the second main aspect of the present invention, there is provided a power control method for a CDMA reception apparatus having a plurality of fingers and serving to perform RAKE reception by RAKE-combining outputs from the plurality of fingers, comprising the steps of detecting a multi-path consisting of a plurality of radio waves arriving with some delay times by despreading a reception signal, and generating a delay profile by obtaining reception power levels of all reception signals in the multi-path, wherein (a) a difference di in level between a signal having an ith highest level and a signal having an (i+1)th highest level of data of the delay profile is obtained, and operation of a finger corresponding to a signal having a low level is stopped on the basis of the difference di and a predetermined threshold dTH, (b) operation of a finger corresponding to a signal having a low level is stopped on the basis of a result obtained by comparing a threshold LTH with a level Li of a signal having an ith highest level of the data of the delay profile, or (c) if a level of a signal having a highest level of the data of the delay profile is not more than a predetermined threshold LMAX, operation of all fingers is stopped.
As is obvious from the above aspects, according to the present invention, when signals arriving through a plurality of radio waves in a multi-path are arranged in the descending order of signal levels, the level difference between adjacent signals is obtained, and the level difference is compared with a threshold. This makes it possible to detect xe2x80x9ca signal group having levels similar to each other to some degreexe2x80x9d in the prior art. Hence, RAKE reception can be performed by using signals that provide a great diversity effect, and reception quality can be guaranteed even in a weak electric field as much as possible.
According to the present invention, RAKE reception is performed without using signals having low levels by which only a small diversity effect can be obtained. With this operation, operation of unnecessary fingers is stopped. As a consequence, the power consumption can be reduced.
According to the present invention, therefore, a RAKE reception effect can be obtained, and power consumption can be optimized.
In addition, according to the present invention, if the level of the signal, of signals arriving through a plurality of radio waves in a multi-path, which has the highest level equal to or lower than the threshold LMAX, all the fingers are operated. This makes it possible to obtain relatively high reception quality even in a weak electric field.